The present invention relates to a method and an apparatus for detecting a spectrum spread signal of an additional information signal superimposed on a main information signal of an image signal by being spectrum-spread.
Digital information apparatuses such as a digital VTR (Video Tape Recorder), MD (Minidisc) recording and reproducing apparatuses have been spread and further, DVD (Digital Video Disc or Digital Versatile Disc) apparatuses having recording function have shown up. According to the digital information recording apparatuses, various additional information signals can be recorded by being added to a digital image signal or a digital audio signal as well as data for a computer or the like as a major information signal.
FIG. 1 shows an example of a constitution of a conventional recording apparatus for recording an additional information signal by adding it to a digital information signal. According to the recording apparatus in FIG. 1, a digital information signal Vi is supplied to a recording apparatus 10 via an input terminal 11 and in the recording apparatus 10, an additional information signal is added to the digital information signal Vi and stored to a record medium 15.
According to the recording apparatus 10, at an adding unit 12, the additional information signal from an additional information generating unit 13 is added to the digital information signal Vi. In this case, the additional information signal is a digital signal and is added to the digital information signal Vi by being recorded to an area which is differentiated from the digital information signal in view of area of, for example, a header portion added to data of a block unit, or other area of TOC (Table of Contents) in the digital information signal.
The digital information signal added with the additional information signal from the adding unit 12 is recorded on the record medium 15 such as a tape or a disc by a writing unit 14. In this case, when compressed code formation of the digital information signal is needed as mentioned above, a processing of the compressed code formation is carried out at the writing unit 14.
As described above, in the case of the conventional recording system of additional information signal, the additional information signal is not directly superimposed on the digital information signal of the header portion or the like but is added to an indirect portion. Therefore, the additional information signal is removed comparatively easily by filtering or alteration and in a recording apparatus or a reproducing apparatus, necessary additional information signal may not be detected. Particularly, when as an additional information signal, control information for preventing unfair copying, copy right information or the like is added, for removing such an additional information signal, a state where an original object cannot be achieved is brought about.
Further, in the case where an additional information signal is added to an indirect portion as mentioned above, when a digital information signal is converted into an analog signal, only a main information signal is obtained and therefore, the additional information signal is detached. Thereby, even when a copying preventive control signal as mentioned above is superimposed by which a countermeasure capable of preventing a digital information signal from being unfairly copied, is provided, when the digital signal is converted into an analog signal, the copying preventive countermeasure is no more effective.
As a system of superimposing an additional information signal for resolving the problem of detachment of the additional information signal and the problem which is caused when the digital signal is converted into an analog signal as mentioned above, the applicant has proposed a system where an additional information signal such as a copying preventive control signal or the like is spectrum-spread and the spectrum-spread additional information signal is superimposed on an image signal and the image signal is recorded by digital recording or analog recording (U.S. patent application Ser. No. 08/75510).
According to the system, a code of PN (Pseudorandom Noise) series which is used as a spread code, is generated at sufficiently high rate and is multiplied with an additional information signal to thereby spectrum-spread the code and the additional information signal of a copying preventive control signal or the like having a narrow band at a high level is converted into a signal having a wide band at a very small level which does not effect influence on an image signal or a vocal signal. Further, the additional information signal which has been spectrum-spread, that is, the spectrum spread signal is superimposed on an analog image signal and is recorded in a record medium. In this case, either of an analog and a digital signal is feasible as an image signal recorded in a record medium.
According to the system, an additional information signal of a copying preventive control signal or the like is spectrum-spread and is superimposed on an image signal as a signal at a very small level and therefore, it is difficult for, a person trying to illegally copy the signal to remove the superimposed copying preventive control signal from the image signal.
In the meantime, the additional information signal such as a superimposed copying preventive control signal or the like can be detected and utilized by subjecting the signal to inverse spectrum spreading. Accordingly, for example, not only is the copying preventive control signal firmly provided to the side of a recording apparatus along with the image signal but also on the side of the recording apparatus, the copying preventive control signal can be detected and decoding control, in accordance with the detected copying preventive control signal, can firmly be carried out.
As mentioned above, in reproducing the main information signal, the additional information signal superimposed on the main information signal such as an image signal, cannot be removed from the main information signal and accordingly, a spectrum spread signal needs to superimpose at a very small level which does not effect influence on a reproduced output of the main information signal.
However, when the spectrum spread signal is superimposed on the main information signal at such a very small level, it is important that the spectrum spread signal can be detected firmly from the main information signal.
An evaluation function  in the case of inverse spectrum spreading where a spectrum spread signal is detected from a signal Si produced by superimposing a spectrum spread signal on a main information signal is specified as follows.
=xcexa3sixc2x7pi
=xcexa3(Vi+kixc2x7pi)pi
=xcexa3Vixc2x7pi+xcexa3kixc2x7pixc2x7pixe2x80x83xe2x80x83(1)
where notation Vi designates a main information signal such as an image signal or the like, notation pi designates a spread code such as a PN code or the like and notation ki designates a coefficient, respectively.
In Equation (1), the first term represents correlation between the main information signal and the spread code and the second term represents correlation between the spectrum spread signal and the spread signal. It is known from Equation (1) that when there is no correlation between the main information signal and the spread signal, the spectrum spread signal can easily be detected.
However, actually, the main information signal such as an image signal or the like is not uncorrelated with the spread code. That is, when the main information signal is a flat signal having no change, the main information signal is not correlated with the spread code, however, a general main information signal is changed in accordance with content of information and therefore, it cannot be uncorrelated with a PN code.
Therefore, in the case where a spectrum spread signal superimposed on a main information signal is deteriorated by, for example, undergoing influence of a low pass characteristic of a transmission path, the correlation between the spectrum spread signal and the PN code becomes low, influence of a degree of correlation between the main information signal and the PN code becomes large and the spectrum spread signal may be difficult to detect.
It is an object of the present invention in view of the above-described points to provide a method and an apparatus capable of firmly detecting a spectrum spread signal even in the case where the spectrum spread signal is deteriorated and a correlation value between the spectrum spread signal and a spread code becomes low.
According to an aspect of the present invention, there are provided a method and an apparatus for detecting an additional information signal from a main information signal with additional information which is formed by superimposing a spectrum spread signal which is formed by spectrum-spreading the additional information signal by a spread code generated at a period in synchronism with a synchronizing signal on the main information signal including the synchronizing signal wherein a spread code the same as the above-described spread code is generated in synchronism with the synchronizing signal included in the main information signal with additional information, the main information signal with additional information is subjected to inverse spectrum spreading by the spread code, a correlation value is formed, the correlation value is compared with a predetermined threshold level, when the correlation value is larger than the predetermined threshold level, the additional information signal is detected from the main information signal with additional information, when the correlation value is smaller than the predetermined threshold level, the correlation value is added at every period during a plurality of periods of the spread code, the threshold level is increased from the predetermined threshold by a predetermined amount of increase at every period of the spread code and when the added correlation value becomes larger than the increased threshold level, the additional information signal is detected from the main information signal with additional information.
In this way, in the case where a spectrum spread signal superimposed on an information signal such as an image signal or the like cannot be detected, correlation values each of which is calculated at every section of a synchronizing period, are added and a threshold for detecting correlation is rendered an appropriate value in accordance with a number of the sections of the synchronizing period which become an object of adding the correlation values by which the spectrum spread signal which cannot be detected by deterioration can be detected.
Thereby, even in the case where the spectrum spread signal superimposed on the image signal is deteriorated and the spectrum spread signal cannot be detected, the spectrum spread signal can firmly be detected.